System for controlling the transmission power of a base station with which a number of mobile stations are in communication

ABSTRACT

The present invention concerns a system for controlling the transmission power of a base station with which a number of mobile stations are in communication, said base station having power command units which receive respectively the signals intended for said mobile stations and power command signals sent by said mobile stations for commanding the contribution of said signal to the transmission power of said base station, and a summer ( 10 ) for forming a composite signal from said signals delivered by said power command units.  
     According to the invention, said system is characterised in that said base station has summation units, the input signals of a given priority level being applied to the inputs of each summation unit which then delivers a sub-composite signal to the input of an attenuation unit whose output is connected to the input of said summer, each attenuation unit being designed to attenuate, by a variable attenuation coefficient, the sub-composite signal at its input when the transmission power of said base station exceeds a predetermined power, said attenuation coefficient being larger the lower the corresponding input signal priority level.

[0001] The present invention concerns a system for controlling thetransmission power of a base station with which a number of mobilestations are in communication.

[0002] In a telecommunications system for mobile telephones (or mobilestations), the signal which is transmitted by a base station serving acell is a composite signal composed of the sum of the signalsrespectively dedicated to the mobile stations which are situated in thecoverage of said cell and specific signals intended for all the mobilestations. All these signals are sent multiplexed. For example, as is thecase in a CDMA telecommunications system, all these signals are sent atthe same time, at the same frequency, multiplexed by means of spreadcodes.

[0003]FIG. 1 depicts schematically the sending section of a base stationof such a telecommunication system. It can be noted that the signals e₁to e_(N) intended for the different mobile stations are summed in asummation unit 10 and that the sum signal is converted intoradiofrequencies in a conversion unit 11 before being amplified by anamplifier 12 in order to be sent to mobile stations attached to saidbase station by means of an antenna 13.

[0004] In such a telecommunications system for mobile telephones, powercontrol means are also provided so that the signal received by a mobilestation is of constant power, despite the attenuation of the propagationchannel between it and the base station. For example, each mobilestation SM_(i) sends a command signal TPC_(i) which represents a requestfor increase or decrease, on the part of the base station, of thetransmission power intended for it. In FIG. 1, power control units 14 ₁to 14 _(N) are provided for respectively receiving the command signalsTPC₁ to TCP_(N) sent by mobile stations SM₁ to SM_(N) and for modifying,before their application to the corresponding inputs of the summationunit 10, the input signals e₁ to e_(N) in accordance with the requestsof these command signals TPC₁ to TCP_(N). Thus, the total power of thecomposite signal transmitted by the base station is the sum of thepowers of the various signals (dedicated and common) which compose it.This power varies over time and depends at each instant on the number ofmobile stations attached to said base station.

[0005] Furthermore, it should be noted that, in such atelecommunications system, when the number of mobile stations attachedto a base station becomes larger and larger, the overall level ofinterference between mobile stations increases. In this case, in orderto keep the signal to interference ratio constant (which ensures a goodquality of service level), the transmission power level of all themobile stations is generally increased. Consequently, theoretically, thenecessary total power level transmitted by the base station divergestowards infinity when the number of mobile stations attached to a basestation approaches a limit value which is referred to as the maximumcapacity of the base station.

[0006] The same phenomenon can be observed when the mobiles move awayoverall from the base station. The transmission power of the latter mayalso increase drastically.

[0007] However, the power which can be supplied by the power amplifier12 of a base station of such a telecommunications system is necessarilylimited, so that, when the number of mobile stations attached to saidbase station increases or when the mobiles move away overall from thebase station, its output signal becomes saturated. What is more, thenon-linearity of this amplifier causes a high distortion of the outputsignal before even reaching this saturation level. This leads to a breakin communications for all the mobile stations against which it isimportant to protect.

[0008] A consequence of this is also a limitation of the coverage of thebase station concerned.

[0009] In order to solve the problem which has just been explained,means have been proposed to ensure that the mean transmission power ofthe composite signal is always less than a critical predetermined valuereferred to subsequently as the operating power value. This operatingvalue is for example equal to the saturation value of the poweramplifier 12 decreased by a reduction value taking into accountinstantaneous fluctuations in the power of the composite signal aroundits mean value.

[0010] One known solution for implementing these means consists of usinga power limiter which, at the time of saturation of the base station,acts on the composite signal so that its transmission power, afteramplification, is less than a predetermined power. Yet this solution hasthe effect of uniformly affecting all the elementary signals forming thecomposite signal. Distortion of the composite signal is certainlyavoided but the contribution to the transmitted power intended for aparticular mobile station is necessarily less than that required by saidmobile station. Thus, the drawback of such a solution lies in the factthat all the mobile stations are uniformly adversely affected in thequality of their transmission.

[0011]FIG. 2 depicts a system which uses means allowing theimplementation of another known solution to the problem mentioned above.The same units as those which have already been depicted in FIG. 1 havethe same references. This system has, in addition, limitation units 15 ₁to 15 _(N) which act respectively on the elementary signals forming thecomposite signal. These units 15 ₁ to 15 _(N) are, for example, as isthe case in FIG. 2, placed between the power control units 14 ₁ and 14_(N) and the corresponding inputs of the summation unit 10.

[0012] The function of each unit 15 _(i) is to modify the signal on itsinput so as to limit its mean contribution Pi to the power of thetransmitted composite signal to a predetermined value Pi_max, the sum ofthese values Pi_max always being less than or equal to the value of thedesired operating transmission power.

[0013] By virtue of this system, the elementary signals intended for onemobile station can saturate, without, for all that, having an impact onthe elementary signals of the other mobile stations. Thus, in the eventof saturation of the signals intended for a mobile station, the level ofpower it receives will not be sufficient to maintain the communicationwhich will then be interrupted. However, the other mobile stations whoseelementary signals are not saturated do not have to experience a breakin communication.

[0014] One drawback of this procedure lies in the fact that it isnecessary, upon the establishment of a new communication for a mobilestation, to decide on the maximum value Pi_max allowed for itscontribution to the transmission power of the base station to which itis attached, which is not in itself trivial since any decision hasimportant repercussions on the capacity and extent of the coverage ofthe base station concerned. Furthermore, it is also still necessary tomake sure that the sum of the maximum transmission powers Pi_max of thededicated signals is definitely always less than the value of theoperating power of the base station. This requires a planning unit inthe network which proves costly and difficult to maintain, especiallywhen communications are switched from one base station to another duringmovement of the mobile.

[0015] It should be noted that this procedure is based on the fact thatthe power of the composite signal is equal to the sum of the powers ofthe elementary signals forming this composite signal. However, thecontribution of power of each elementary signal to the transmissionpower of the base station being limited by its maximum power Pi_max, thetransmission power of the composite signal is limited by the sum ofthese maximum power contributions. In practice, the power contributionsof the elementary signals are not all at their maximum values at thesame time. For example, only the signals intended for the mobilestations at the edge of the coverage circle can be at their maximumpower level at the same time, whereas the signals intended for themobile stations close to the base station will be transmitted at a lowerpower value. Thus, even at high load, the power of the composite signalis a long way from reaching its operating value, which will thereforepractically never be used.

[0016] The aim of the present invention is therefore to propose aprocedure which does not have the drawbacks mentioned above of theprocedures of the state of the art. In particular, it is to propose aprocedure which makes it possible to make best use of the power of thebase station while avoiding phenomena of distortion, under-use of theradio capacity, and too weighty power planning installations in thenetwork.

[0017] To that end, the present invention concerns a system forcontrolling the transmission power of a base station with which a numberof mobile stations are in communication, said base station having powercommand units which receive respectively the signals intended for saidmobile stations and power command signals sent by said mobile stationsfor commanding the contribution of said signal to the transmission powerof said base station, and a summer for forming a composite signal fromsaid signals delivered by said power command units. This control systemis characterised in that said base station has summation units, theinput signals of a given priority level being applied to the inputs ofeach summation unit which then delivers a sub-composite signal to theinput of an attenuation unit whose output is connected to the input ofsaid summer, each attenuation unit being designed to attenuate, by avariable attenuation coefficient, the sub-composite signal at its inputwhen the transmission power of said base station exceeds a predeterminedpower, said attenuation coefficient being larger the lower thecorresponding input signal priority level.

[0018] According to another characteristic of the invention, saidattenuation coefficient of each attenuation unit is a power P of a baseattenuation coefficient, the value of P being identical for all saidattenuation units.

[0019] According to another characteristic of the invention, each baseattenuation coefficient is less than unity, the coefficient of anattenuation unit being closer to unity the higher the correspondinginput signal priority level.

[0020] According to another characteristic of the invention, an inputsignal intended for a mobile station is assigned to a sub-compositesignal at the beginning of the communication.

[0021] According to another characteristic of the invention, theassignment of an input signal intended for a mobile station to asub-composite signal is modified only at the time of the arrival of atleast one event which concerns only said mobile station.

[0022] According to another characteristic of the invention, said oreach event is a change in type of service, reaching of the saturationlevel, or entry into soft handover of said mobile station.

[0023] The present invention also concerns a method of controlling thetransmission power of the sending section of a base station whichreceives, on its inputs, input signals which are intended to betransmitted to mobile stations attached to said base station. Accordingto the invention, such a method is characterised in that it consists offorming groups of input signals according to predetermined prioritycriteria assigned to said input signals and of forming, from said inputsignals of each group, sub-composite signals, of forming, from saidsub-composite signals, a composite signal which is transmitted to saidmobile stations, and in that it consists of attenuating saidsub-composite signals so that the power of the composite signal isalways less than a predetermined power level, the attenuationcoefficients respectively applied to said sub-composite signals beingdifferent according to the sub-composite signals considered.

[0024] According to another characteristic of the invention, saidattenuation coefficient applied to each sub-composite signal has a valuewhich is greater the higher the priority level of the input signals fromwhich it is formed.

[0025] According to another characteristic of the invention, theattenuation coefficients respectively applied to the sub-compositesignals are a same power P of base attenuation coefficients, thevariation of said attenuation coefficients being obtained by variationof said value of the power P.

[0026] According to another characteristic of the invention, the valueof P is chosen so as to allow non-exceeding of the critical power level,the value just below, P−1, causing exceeding of said critical powerlevel.

[0027] According to another characteristic of the invention, each baseattenuation coefficient is less than unity, the coefficient of anattenuation unit being closer to unity the higher the correspondinginput signal priority level.

[0028] According to another characteristic of the invention, an inputsignal intended for a mobile station is assigned to a group for forminga sub-composite signal at the beginning of the communication.

[0029] According to another characteristic of the invention, theassignment of an input signal intended for a mobile station to a groupfor forming a sub-composite signal is modified only at the time of thearrival of at least one event which exclusively concerns only saidmobile station.

[0030] According to another characteristic of the invention, said oreach event is a change in type of service, reaching of the saturationlevel, or entry into soft handover of said mobile station.

[0031] The characteristics of the invention mentioned above, as well asothers, will emerge more clearly from a reading of the followingdescription of an example embodiment, said description being given inrelation to the accompanying drawings, among which:

[0032]FIG. 1 is a schematic view of a communication system for mobiletelephones, in particular of the sending section of a base station andof mobile stations,

[0033]FIG. 2 is a schematic view of the sending section of a basestation according to a second embodiment in accordance with the priorart,

[0034]FIG. 3 is a schematic view of the sending section of a basestation according to the present invention.

[0035]FIG. 3 shows the sending section of a base station SB whichreceives, on its inputs, input signals e₁ to e_(N) which are intended tobe transmitted to mobile stations SM₁ to SM_(M) attached to said basestation SB. These input signals e₁ to e_(N) can be signals dedicated toany one mobile station but also signals intended for a set or for allthe mobile stations SM₁ to SM_(M).

[0036] According to the method of the present invention, groups of inputsignals are formed according to predetermined priority criteria assignedto these signals and the input signals of each group are summed so as toform sub-composite signals. The sub-composite signals thus formed aresubject to respective attenuation processings before being summed toform a composite signal which is transmitted to said mobile stations.The attenuation processings implemented are designed so that the powerof the composite signal is always less than a predetermined powerreferred to as the operating power of said base station, the attenuationcoefficients being different according to the composites signalsconsidered.

[0037] Thus, when the mean power of the composite signal becomes greaterthan the operating power, modifications can be made, different one fromanother, to the contributions of the sub-composite signals to thetransmission power of the composite signal, that is to say thetransmission power of the base station.

[0038] For the implementation of the method of the present inventionwhich has just been explained, the sending section whose diagram isdepicted in FIG. 3 is proposed.

[0039] This sending section has as many power control units 14 ₁ to 14_(N) as input signals e₁ to e_(N) which it is capable of sending to Mmobile stations SM₁ to SM_(M). It has summation units 15 ₁ to 15 _(K) soas to form K sub-composite signals Sc₁ to SC_(K) respectively applied toattenuation units 16 ₁ to 16 _(K).

[0040] On the inputs of each summation unit 15 ₁ to 15 _(K) there areapplied input signals, forming a group of input signals, selectedaccording to priority criteria. Each sub-composite signal is thereforeassigned to a priority level.

[0041] On one input of each attenuation unit 16 ₁ to 16 _(K), there isapplied the attenuation coefficient α₁ to α_(K) to which thesub-composite signal on its input is subjected. This thus gives thesignal at the output sc_(i) of an attenuation unit 16 _(i) which can beexpressed as follows:

sc _(i)=α_(i) ×Sc _(i)

[0042] Advantageously, each attenuation coefficient α_(i) has a valuewhich is between 0 and 1.

[0043] Furthermore, the higher the priority level of a sub-compositesignal Sc_(i), the closer the coefficient α₁ is to 1 and vice versa, thelower the priority level of a sub-composite signal Sc_(i), the closerthe coefficient α_(i) is to 0.

[0044] Once attenuated, the sub-composite signals sc_(i) are applied toa summation unit 10 intended to form a composite signal Sc. As in thestate of the art, the composite signal Sc is converted intoradiofrequencies in a conversion unit 11 before being amplified by anamplifier 12 in order to be sent by means of an antenna 13.

[0045] With such a sending section, when, at a time t, the mean power ofthe composite signal becomes greater than a critical level, all theattenuation coefficients α₁ to a_(K) are reduced, for examplediscretely, until a time when the power of the composite signal Scbecomes lower than the predetermined operating power.

[0046] Advantageously, the higher the priority level of a sub-compositesignal sc_(i), the less the coefficient α_(i) is reduced and vice versa,the lower the priority level of a sub-composite signal sc_(i), the morethe coefficient α_(i) is reduced.

[0047] Still advantageously and for implementation of the previouscharacteristic, the effective attenuation α_(i) applied to asub-composite signal Sc_(i) is equal to a P^(th) power of a baseattenuation α_(I) ⁰, P being identical for the K sub-composite signalsSc_(i) to Sc_(K) (this therefore gives α_(i)=(α_(i) ⁰)^(P)).

[0048] Moreover, the value of P is greater than or equal to zero. Thus,it is by varying the value of P that the contribution to thetransmission power of each sub-composite signal will be varied. Thisvalue of P is determined so that the power of the composite signal issubstantially equal to the level of the operating power.

[0049] For example, the value of P will be chosen which results in theoperating power not being exceeded but such that the choice of the valueof P−1 in its place would have resulted in this operating power beingexceeded.

[0050] It should be noted that, with this type of procedure, certainpriority calls may never be saturated (for example where the basecoefficient α_(i) ⁰ is chosen equal to one). This procedure neverthelessmakes it possible that non-priority calls can, all the same, getthrough, for example when the communication can be carried out by meansof another non-saturated base station (the case of a call in softhandover), or when the communication can be carried out a little later.

[0051] According to the embodiment depicted in FIG. 3, the system of thepresent invention has a checking unit 17 which receives, on its input,the composite signal issuing from the summer 10 and which delivers thevalue of P to an arithmetic unit 18 designed to determine thecoefficients α₁ to α_(K) from base coefficients α₁ ⁰ to α_(K) ⁰.

[0052] When the composite signal Sc has a power which exceeds theoperating power, the checking unit 17 increments the value of P, theconsequence of which is an increasing of the attenuation coefficients α₁to α_(K) and therefore of the sub-composite signals sc₁ to sc_(K). Whenit is at a level less than this critical level, it decrements the valueof P, the effect of which is a decreasing of the attenuationcoefficients α₁ to α_(K) and therefore of the sub-composite signals sc₁to sc_(K).

[0053] The allocation, to a given call, of one priority level ratherthan another and, consequently, the allocation of the input signalse_(i) which are intended for it, to a group for forming a sub-compositesignal sc_(j), will be carried out by considering the calls already insaturation and the calls which are not, by differentiating callsaccording to the number of base stations involved in their soft handoverphases and/or by differentiating connection type calls from packet typecalls.

[0054] According to another method of the invention, a call is assignedto a priority level at the beginning of the communication. Itsmembership of one group for forming a sub-composite signal rather thananother is modified only according to exceptional events whichexclusively concern only said call. These may be, for example, a changein type of service, the arrival at its saturation level of said call,its entry into soft handover phase, the variation in the number of basestations to which the corresponding mobile station is attached inhandover phase, etc. The calls are therefore assigned to a sub-compositesignal independently of what takes place for the other calls.

[0055] The present invention allows an optimum effective use of thepower of the base station when it becomes saturated. It does notrequire, unlike the systems of the state of the art, rapid refreshing ofthe maximum power levels among all the signals, nor a jointdetermination of these levels. This is because the base coefficients aredetermined in advance by the network operator.

1. System for controlling the transmission power of a base station withwhich a number of mobile stations are in communication, said basestation having power command units which receive respectively thesignals intended for said mobile stations and power command signals sentby said mobile stations for commanding the contribution of said signalto the transmission power of said base station, and a summer (10) forforming a composite signal from said signals delivered by said powercommand units, characterised in that said base station has summationunits, the input signals of a given priority level being applied to theinputs of each summation unit which then delivers a sub-composite signalto the input of an attenuation unit whose output is connected to theinput of said summer, each attenuation unit being designed to attenuate,by a variable attenuation coefficient, the sub-composite signal at itsinput when the transmission power of said base station exceeds apredetermined power, said attenuation coefficient being larger the lowerthe corresponding input signal priority level.
 2. System for controllingthe transmission power of a base station according to claim 1 ,characterised in that said attenuation coefficient of each attenuationunit is a power P of a base attenuation coefficient, the value of Pbeing identical for all said attenuation units.
 3. System forcontrolling the transmission power of a base station according to claim2 , characterised in that each base attenuation coefficient is less thanunity, the coefficient of an attenuation unit being closer to unity thehigher the corresponding input signal priority level.
 4. System forcontrolling the transmission power of a base station according to one ofthe preceding claims, characterised in that an input signal intended fora mobile station is assigned to a sub-composite signal at the beginningof the communication.
 5. Control system according to one of thepreceding claims, characterised in that the assignment of an inputsignal intended for a mobile station to a sub-composite signal ismodified only at the time of the arrival of at least one event whichconcerns only said mobile station.
 6. Control system according to claim5 , characterised in that said or each event is a change in type ofservice, reaching of the saturation level, or entry into soft handoverof said mobile station.
 7. Method of controlling the transmission powerof the sending section of a base station SB which receives, on itsinputs, input signals e₁ to e_(N) which are intended to be transmittedto mobile stations SM₁ to SM_(M) attached to said base station SB,characterised in that it consists of forming groups of input signalsaccording to predetermined priority criteria assigned to said inputsignals and of forming, from said input signals of each group,sub-composite signals, of forming, from said sub-composite signals, acomposite signal which is transmitted to said mobile stations, and inthat it consists of attenuating said sub-composite signals so that thepower of the composite signal is always less than a predetermined power,the attenuation coefficients respectively applied to said sub-compositesignals being different according to the sub-composite signalsconsidered.
 8. Control method according to claim 7 , characterised inthat said attenuation coefficient applied to each sub-composite signalhas a value which is greater the higher the priority level of the inputsignals from which it is formed.
 9. Control method according to claim 8, characterised in that the attenuation coefficients respectivelyapplied to the sub-composite signals are a same power P of baseattenuation coefficients, the variation of said attenuation coefficientsbeing obtained by variation of said value of the power P.
 10. Controlmethod according to claim 9 , characterised in that the value of P ischosen so as to allow non-exceeding of said predetermined power, thevalue just below, P−1, causing exceeding of said predetermined power.11. Method for controlling the transmission power of a base stationaccording to claim 9 or 10 , characterised in that each base attenuationcoefficient is less than unity, the coefficient of an attenuation unitbeing closer to unity the higher the corresponding input signal prioritylevel.
 12. Method for controlling the transmission power of a basestation according to one of claims 7 to 11 , characterised in that aninput signal intended for a mobile station is assigned to a group forforming a sub-composite signal at the beginning of the communication.13. Control method according to one of claims 7 to 12 , characterised inthat the assignment of an input signal intended for a mobile station toa group for forming a sub-composite signal is modified only at the timeof the arrival of at least one event which exclusively concerns onlysaid mobile station.
 14. Control method according to claim 13 ,characterised in that said or each event is a change in type of service,reaching of the saturation level, or entry into soft handover of saidmobile station.